This invention relates to a workpiece holder and more particularly, to a workpiece holder capable of securing annularly shaped workpieces of varying diameters.
A turbomachine, such as an industrial gas turbine for a co-generation system or a gas turbine engine for an aircraft, includes a compressor section, a combustion section, and a turbine section. The compressor section and more particularly, the low pressure compressor section is constructed of a series of circular discs having diameters ranging from about 2.5 ft. (0.75 meters) to 5.0 ft. (1.5 meters). These discs have an annular shape, and therefore, are often referred to as rings. Blades are attached to the exterior circumference of these discs, and as the circular discs rotate, the blades direct the flow of air through the compressor section.
In order to affix the blades to the disc""s exterior circumference, precision formed axial slots are machined to within very small tolerances on the disc""s exterior circumference, and the blades are inserted within the slots. Thus, the exterior circumference of the disc contains a series of machined slots. These slots are typically formed by a broaching process. Specifically, the broaching machine forms each slot by progressively tearing away (i.e., shearing) away multiple layers of material until the desired shape is achieved. The high force, ranging from about 3000 to 5000 pounds, required to tear away each layer results in creating undesirable stress in and around the slot.
Broaching machines, as well as the fixtures used to secure the discs within the broaching machine, are typically quite large and consume a significant amount of floor space. Because the size of the discs and fixtures are relatively large, the broaching process is susceptible to distorting the discs during set-up or actual machining. Specifically, when the disc is secured within the broaching fixture, the fixture may be over tightened by the operator, thereby causing the edges of the disc to flare. Hence, when the slots are machined-in the edges of such disc, the resulting slot location may not be in the originally specified position. Thus, when multiple discs are attached to each other to form a compressor section, the blades within the consecutively spaced discs may not align with each other.
The broaching process is also very time consuming. Moreover, the procedure for setting-up the broaching machine to manufacture slots in circular discs is complicated, thereby further increasing the overall manufacturing time. Furthermore, a different broaching fixture must be used for each distinctly shaped disc. In other words, if the interior or exterior diameter of the disc varies from another disc, a different fixture must be used. Because a compressor section typically includes a variety of discs having different diameters, numerous broaching fixtures exist and consume significant floor space. Moreover, each broaching fixture is expensive. Therefore, the tooling cost to manufacture a compressor section of a turbomachine is costly. Overall, broaching is a time consuming and expensive method for manufacturing slots within circular discs.
One alternative to broaching is milling, which is a process wherein a mechanical bit rotates and gradually slices or scoops small portions of material until the desired shape is achieved. Compared to the broaching process, milling imparts less residual stress in and around the slot because the bit rotates while moving through the slot. Additionally, the milling tool cuts the workpiece with about 100 to 300 pounds of force, which is only a fraction of the force required in the broaching process. Milling machines are also typically smaller than broaching machines and consume less floor space. Milling is also typically a less time consuming machining process and easier to set-up compared to broaching. Conventional milling fixtures, however, include a restraint, which secures the workpiece in place. This restraint increases the complexity of aligning the bit with the circular disc, which could prevent the milling machine from machining all of the necessary slots within the circular disc in one process. Moreover, in order to mill the slots within one circular disc, two different conventional milling fixtures must be used. One fixture is used during the milling process and the other fixture is used to hold the workpiece during inspection. Therefore, for each differently sized disc, two milling fixtures are required. As mentioned above, a turbomachine""s compressor section typically comprises a plurality of discs. Hence, the tooling cost to mill the blade slots within a compressor section is comparable to the tooling cost to broach such slots.
What is needed is a fixture that is capable of securing various sized annularly shaped workpieces within a milling machine.
The present invention is a universal workpiece holder that includes a unique positioning hub and a plurality of clamping devices for clamping a variety of annular workpieces having different interior and/or exterior diameters. The unique configuration of the positioning hub, which includes multiple sets of spokes extending radially outward from a circular hub, can rotate about its center such that when a certain set of spokes are circumferentially aligned with the clamping devices, the workpiece holder can clamp and secure a corresponding annular workpiece therein. Moreover, the positioning hub can thereafter further rotate about its center such that a different set of spokes are circumferentially aligned with the clamping devices, thereby providing the workpiece holder with the ability to clamp an annulus having a different interior and/or exterior diameter. This versatile workpiece holder eliminates the requirement of having a separate workpiece holder for each distinctly shaped annulus. Rather, individual workpiece holders are replaced with different sets of spokes, whereby each set of spokes has a different length. The spokes cooperate with the clamping devices such that the diameter of the clamping devices is approximately equal to the inside diameter of different annuluses. Once the clamping devices are circumferentially aligned with the appropriate set of spokes, the clamping devices abut the spokes and clamp both the interior and exterior of the annulus at circumferentially aligned opposing points.
The spokes within each set of spokes are equiangularly spaced about the hub. Therefore, when the clamping devices abut the spokes and clamp the annulus, it is clamped with an even distribution of force, thereby reducing the possibility of distorting the annulus. In addition to minimizing the possibility of distortion and reducing the amount of equipment needed to machine different sized annular workpieces and reducing the necessary space associated therewith, the workpiece holder of the present invention is also less complex than those which currently exist. Removing the complexity involved in changing the fixture for each distinctly shaped disc reduces the overall set up time, which, in turn, increases the efficiency of the overall machining process.
Accordingly the present invention relates to a universal workpiece holder for securing a variety of differently sized annular workpieces therein, wherein the annular workpieces have an interior and exterior diameter, the universal workpiece holder including a circular hub having a bore through its center, thereby allowing the circular hub to rotate thereabout, the circular hub including a first plurality of spokes extending radially outward from the circular hub, wherein the first spokes are equiangularly spaced, each of the first spokes having a first predetermined length and a distal end, thereby forming a first hub diameter, and a plurality of clamping devices for circumferential alignment with the first spokes, each of the clamping devices including a sliding block having a locating end, an extensible clamping end, and a length equal to the distance between the locating end and the extensible clamping end, the sliding block positioned outwardly of the circular hub such that the locating end is closer to the circular hub than the clamping end, means for allowing the first block to travel radially of the hub such that when the locating end abuts the distal end of the spoke, the sum of the first hub diameter and twice the length of the sliding block is approximately equal to the interior diameter of the annulus, means for extending the extensible clamping end radially outward of the circular hub for pressing the extensible clamping end against the interior diameter of the annulus, and clamping means for applying a force against the exterior diameter of the annulus at a location circumferentially aligned with the sliding block for clamping the annulus between the clamping end of the sliding block and the clamping means.
In an alternate embodiment of the present invention, the positioning hub further includes additional sets of spokes that are equiangularly spaced from one another but differ in length from the original spokes. The different spoke lengths create a different imaginary diameter and circumference than the original spokes created, thereby accommodating an annularly shaped workpieces having a different diameter. Specifically, the sum of the diameter of the positioning hub, the length of two spokes, and the length of two sliding blocks is about equal to the interior diameter of the annularly shaped workpiece.
The positioning hub, comprising numerous pairs of spokes, can rotate about its center and quickly align with the clamping devices. Once the clamping devices abut the spokes, the clamping devices are ready to receive an annularly shaped workpiece. The positioning hub, therefore, allows one workpiece holder to secure a variety of annular circular shaped discs of different diameters. Thus the present invention is an improvement over the conventional fixtures used for milling because rather than having a different fixture for each disc having a different interior or exterior diameter, the workpiece holder has a positioning hub with different pairs of spokes that correspond to distinctly sized discs. The workpiece holder of the present invention makes milling an attractive alternative for manufacturing slots in annular discs because the workpiece holder is a single fixture that can hold various sized annular discs. A single workpiece holder, therefore, reduces the overall number of fixtures required to manufacture a compressor section, which contains several different sizes of discs. Reducing the overall number of fixtures to manufacture a compressor section, in turn, reduces the tooling cost for such process, which ultimately reduces the overall manufacturing cost.